We had a 17kw Generac standby generator installed last fall - our water heater, clothes dryer, cooktop and furnace are all propane so the generator is piped into that same system from our 500 gallon tank. If our grid power goes out for more than ten seconds, the gen fires up and runs everything. It starts itself once a week automatically for fifteen minutes as a self-test, change the oil once a year on the two cylinder four stroke 900cc engine.
Price was a little over 10K plus tax.
Just as a note, the smaller generators usually have a much shorter operating life. The Generac 40kW models, at least the older ones anyway, use a water cooled Chrysler V6 engine and run at 1800 RPM’s. The smaller air cooled versions are not designed to run a lot hours and typically run at 3600 RPMs making them much louder with a much shorter lifespan. Parts for these proprietary engines are much harder to come by verses a common car engine.
The Generac 40kW model I got was $8800 with a 200 A automatic transfer switch. That was about 5 or 6 years ago though.